1. Field of the Invention
The present invention relates to an optical reception apparatus that receives an optical signal of a multivalue phase modulating format used for an optical transmission system, and a controlling method thereof.
2. Description of the Related Art
Recently, a need for introduction of an optical transmission system corresponding to next-generation 40 gigabit per second (Gbps) is increasing, while transmission distance and frequency availability equivalent to those of an existing 10 Gbps system are required. As a means for realizing this, for example, there has been developed an optical transmission system that applies a multivalue phase modulating format such as Return to Zero-Differential Quadrature Phase Shift Keying (RZ-DQPSK) having excellent Optical Signal Noise Ratio (OSNR) tolerance and nonlinearity tolerance as compared with a Non Return to Zero (NRZ) modulating format, which has been applied in a conventional system corresponding to 10 Gbps or less. Moreover, in addition to the application of the above multivalue phase modulating format, there has been also adopted a technique for improving performance such as long distance transmission and high noise tolerance by performing error correction in an electric stage after an optical signal having an error due to transmission deterioration is photoelectrically converted according to a conventional error correction method by Reed-Solomon code or a new error correction coding method.
FIG. 6 is a diagram showing a configuration example of a known 40 Gbps optical transmission system. In this optical transmission system, a plurality of base stations 110 are connected to each other via an optical transmission line 100, and each base station 110 includes a 40 Gbps router 111 connected with a client (not shown), and an optical transmission device 112 connected to the optical transmission line 100. Between the router 111 and the optical transmission device 112 in each base station 110, a 40 Gbps optical signal having a relatively wide optical spectrum width is transmitted in both directions. Moreover, between the optical transmission devices 112 in opposite base stations 110, a 43 Gbps optical signal having a narrow optical spectrum width with an error correction code is transmitted for a long distance in both directions.
FIG. 7 is a diagram showing a configuration example of the optical transmission device 112 in FIG. 6. In this configuration example, a known framing process by a framer LSI 122 and an error correction code-adding process are executed with respect to the signal that has been photoelectrically converted by a 40 Gbps broadband module 121 that transmits and receives the optical signal to and from the router 111 on the client side. Moreover, a narrow-band optical signal for long distance transmission generated by a 43 Gbps RZ-DQPSK module 123 that performs RZ-DQPSK modulation processing according to the signal processed by the framer LSI 122, is amplified to a required level by an optical amplifier 124 and then transmitted to the optical transmission line 100. The narrow-band optical signal propagated through the optical transmission line 100 and received by the optical transmission device 112 is amplified to the required level by the optical amplifier 124, and is then input to the 43 Gbps RZ-DQPSK module 123 and demodulated, and the error correction process of the received signal is performed by the framer LSI 122. A broadband optical signal generated by the 40 Gbps broadband module 121 according to the signal processed by the framer LSI 122 is output to the router 111 on the client side.
FIG. 8 is a diagram showing a configuration example of a transmission unit in the RZ-DQPSK module 123 in FIG. 7. In the transmission unit, a continuous wave (CW) output from a light source 131 is provided to a phase modulator 132 and an intensity modulator 133, and the phase modulator 132 and the intensity modulator 133 are driven based on an electric signal output from the framer LSI 122, to thereby output an optical signal of an RZ-DQPSK modulating format. Specifically, parallel electric signals output from the framer LSI 122 are subjected to serial signal processing by a serializer 134, and then separated into two flows of data signals DA and DB in a separation circuit 135. By driving the phase modulator 132 by a drive signal generated by driving circuits 136A and 136B according to the respective data signals DA and DB, a DQPSK modulated optical signal is output from the phase modulator 132. Moreover, a clock signal CLK having a frequency corresponding to the data signals DA and DB is output from the serializer 134, and the intensity modulator 133 is driven by a drive signal generated by a driving circuit 137 according to the clock signal CLK, to thereby output the RZ-DQPSK modulated optical signal from the intensity modulator 133.
FIG. 9 is a diagram showing a configuration example of a reception unit in the RZ-DQPSK module 123 in FIG. 7. In the reception unit, the RZ-DQPSK optical signal received from the optical transmission line 100 via the optical amplifier 124 is branched into two, and respectively transmitted to an arm A where a delay interferometer 141A is formed and an arm B where a delay interferometer 141B is formed. The delay interferometer 141A makes a 1-bit time delay component and a π/4 rad phase-controlled component interfere with each other (delay interference), and outputs the interference result as two outputs. Moreover, the delay interferometer 141B makes a 1-bit time delay component and a −π/4 rad phase-controlled component (the phase is shifted from that of the component in the delay interferometer 141A by π/2 rad) interfere with each other (delay interference), and outputs the interference result as two outputs. Output beams from the respective delay interferometers 141A and 141B are received by photoelectric conversion circuits 142A and 142B having a pair of a photodiode and an amplifier, to thereby perform differential photoelectric conversion detection. Then, after output signals from the photoelectric conversion circuits 142A and 142B are provided to a multiplex circuit 143 and multiplexed, the multiplexed signals are provided to a deserializer 144 and subjected to parallel signal processing, and signal-processed signals are transmitted to the framer LSI 122 in the subsequent stage. Moreover, the output signals from the photoelectric conversion circuits 142A and 142B are also respectively provided to mixers 145A and 145B, and phase shift amounts in the respective delay interferometers 141A and 141B are controlled by control circuits 146A and 146B, respectively, so that an opening of an eye pattern in an output waveform of the respective mixers 145A and 145B becomes an optimum state.
As a technique associated with the optical reception apparatus corresponding to the reception unit shown in FIG. 9, for example, Japanese Unexamined Patent Publication No. 2005-80304 is known. In Japanese Unexamined Patent Publication No. 2005-80304, it is proposed, as one method for adjusting a relative delay in the delay interferometer, to monitor a bit error rate (BER) of the signal based on an interference signal generated by the delay interferometer, and adjust the relative delay based on the BER.
However, in the conventional technique for receiving the multivalue phase modulated optical signal such as the above RZ-DQPSK optical signal, there is a problem in that it becomes difficult to perform the error correction precisely by the framer LSI 122 in the subsequent stage, due to a burst error occurring in the reception unit in the RZ-DQPSK module 123.
In other words, the optical signal received by the reception unit in the RZ-DQPSK module 123 is amplified by the optical amplifier 124 (FIG. 7) for compensating a loss caused by the long distance transmission. Therefore, amplified spontaneous emission (ASE) occurring in the optical amplifier 124 is added as broadband optical noise. In the RZ-DQPSK signal added with the optical noise, for example, as shown in a conceptual diagram of FIG. 10, noise is carried on a light emission side corresponding to level “1”, and hence, the signal waveform largely collapses.
As in the configuration example shown in FIG. 9, when the mixers 145A and 145B are used to control the phase shift amount in the delay interferometers 141A and 141B, the control largely depends on the signal waveform, and there is an influence of manufacturing variations of the delay interferometers 141A and 141B. Therefore, the relative delay added to between the optical signals propagating through the arms A and B is not in an optimum state. Accordingly, for example, as shown in a conceptual diagram of FIG. 11, an error rate of the signal before error correction corresponding to the optical signal on the arm A side and an error rate of the signal before error correction corresponding to the optical signal on the arm B side are largely different from each other. The broken line in FIG. 11 indicates an error rate (ideal value) when the relative delay is controlled in the optimum state, and the error rates on the arm A side and on the arm B side agree with each other.
Since the error rates on the arm A side and on the arm B side are different, a burst error in which frequent errors arise intermittently, occurs in the signal multiplexed in the multiplex circuit 143 (FIG. 9). In a general error correction method it is difficult to handle such a burst error, and as a result, for example, as shown by the solid line in FIG. 12, error correction cannot be performed precisely by the framer LSI 122 in the subsequent stage, thereby causing degradation of reception performance.